Since it is known that the tat protein of the HIV will enter the cell in vitro and stimulate the production of HIV products, there is almost certainly a receptor site for it on the cell membrane. Segments of this 86 amino acid protein are being synthesized to determine which segment is responsible for binding to the receptor. This phenomenon will be evaluated by quantitatively measuring the binding force between the various segments and several types of human cells grown in tissue culture. We are using a custom built flow cell and image analysis to determine the average shear force needed to detach the cells. This is accomplished by allowing the cells to attach to the bottoms of a 96-well plate, which will be moved automatically from well to well under computer control.